The first time we meet Rengoku’s father, he’s violent and brash. Shockingly, he’s said to be a former Hashira and one of the most skilled swordsmen of them all. So why did Rengoku’s father quit being a swordsman?
Rengoku’s father chose to quit being a swordsman due to his wife’s passing. He wallowed in self-pity and sake, refusing to mentor his sons’ sword skills.
He admits to putting the sword down within a letter to Tanjiro.
Rengoku’s father explains in a letter his decision to quit
Chapter 81 begins with Rengoku’s father penning a letter to Tanjiro. He apologizes for their first interaction, blaming his violent and unpredictable behavior on the loss of his wife. After Rengoku’s mother passes away, Shinjuro drowns his sorrows in sake and wallows in self-pity.
He thanks Tanjiro for mourning Rengoku and corresponding letters with the youngest son, Kyojuro. The two have been sending letters back and forth for at least four months.
The self-pity that came as a result of losing his wife causes Shinjuro to put down the sword, he explains. Even when he stopped teaching Rengoku, the son read the “Guide to Flame Breathing,” mastering the technique with only three available volumes. Despite losing his mentor to depression, Rengoku is still able to become a Hashira.
It’s due to this depression that Rengoku’s Dad reacts to his son’s death so flippantly. After quitting his son’s training, he probably felt a suspicion that one day Rengoku would perish. The sake encouraged Shinjuro to be aggressive and hide his true feelings when the horrible news was heard.
Shinjuro briefly describes his wife, Ruka, in the letter. The Rengoku children are said to be “wonderful boys” because of their mother’s blood running so strongly within them.
So that’s the answer. Rengoku’s father quit being a swordsman after his wife passed away. He took it hard, wallowing in self-pity and sake. He even refuses to mentor his two young sons in the way of the sword, leaving them to their own devices.